In radio equipment for a narrow-band communication system such as Tone in Band, it is important that a frequency for transmission and receiving is accurate, and sometimes a stability of frequency exceeding that stipulated in the Electric Wave Law is required. For instance, in the 220 MHz zone in the United States, in the Electric Wave law (by the F.C.C) a stability of .+-.1.5 ppm (Approx. .+-.350 Hz) is required, but actually it is impossible to execute communication under good conditions unless the stability is in a range from .+-.50 Hz to .+-.100 Hz, so that a more strict stability of frequency than that stipulated in the Electric Wave Law is required.
FIG. 10 is an explanatory view for illustrating a range of frequency stability required in the Tone in Band system (from .+-.50 Hz to .+-.100 Hz). In the Tone in Band system, to respond to frequency shift due to Rayleigh fading and high speed fluctuation of amplitude, signal processing to remove the influence of these factors is executed referring to TONE (described pilot signal hereinafter) inserted at the center of a band as a reference signal. Namely a pilot signal having a constant value inserted at the center of a band is regarded as being under the influence of Rayleigh fading similar to a case of voice, so a processing for restoring the voice signal to its original form depending on the tone is executed.
However, in case of narrow-band communication, a pilot signal inserted in the band (narrow band) is separated from voice by only several hundreds Hz. For this reason, a band for a pilot filter to separate voice from the pilot signal is in a range of only around .+-.100 Hz. For this reason, if the frequency deviation becomes 100 Hz or more, it becomes impossible to receive a pilot signal correctly, or an aural element is mixed in a pilot signal, which deteriorates quality of communication or generates troubles such as impossibility of communication.
For the reasons as described above, in the Tone in Band system, frequency stability in a range of from .+-.50 Hz to .+-.100 Hz is required.
It is possible to realize the frequency stability as described above by employing a quartz oscillator based on an OVEN system or a reference oscillator employing the advanced digital technology therein (described "digital TCXO" hereinafter).
Also, a requirement concerning the frequency stability of electric waves transmitted from a base station is generally more strict for a mobile station, and in the 220 MHz in the United States described above, the allowable frequency deviation is .+-.0.1 ppm. For this reason, a frequency obtained by multiplying the frequency of electric waves transmitted from a base station as a base station by AFC is often used.
On the other hand, as a reference signal for demodulating received signals or for reducing fading, a pilot signal is often used. In a receiving section in a conventional type of radio equipment, the level of a pilot signal is measured, whether any received signal exists or not is determined, and squelch open/close is executed, thus only required voice signals being outputted and output of unnecessary noise being suppressed.
With the conventional type of radio equipment using a quartz oscillator based on an OVEN system as described above, however, a long time is required until the quarts oscillator is stabilized upon power turn ON, and some time is required for standing by after power is turned ON until the radio equipment becomes available.
With the conventional type of radio equipment using digital TCXO, as the digital TCXO is expensive, the apparatus cost becomes disadvantageously high.
Also in the conventional type of radio equipment, there are various types of causes for frequency deviation such as deviation due to an initial adjustment error, deviation due to temperature characteristics, deviation due to time elapse-related change, deviation due to time elapse-related change in parts of the oscillator, and deviation due to temperature characteristics of DC stabilized power supply due to the oscillator and AFC circuit. Even if a quartz oscillator based on the OVEN system or the digital TCXO is used, a satisfactory frequency stability and frequency precision can not always be achieved.
Especially, the frequency deviation due to deviation because of temperature characteristics is very large, and even if the digital TCXO generally regarded as the device having the highest frequency stability as well as the highest frequency precision is used, it has been fairly difficult to achieve a frequency precision in a range from -30.degree. C. to +75.degree. C.
On the other hand, with the conventional type of radio equipment based on the AFC system, in a narrow-band communication such as Tone in Band, communication is disabled by a slight frequency deviation, so that is impossible to multiply AFC without taking necessary measures such as searching by the estimated maximum frequency deviation. Also, the operation of capturing electric waves from a base station correctly is furthermore difficult in a border zone of a service area where the electric waves are very weak. For this reason, only after power for a mobile station is turned ON and electric waves from a base station are searched and received, it becomes possible to multiply AFC. In other words, some time for waiting is required after a mobile station turns ON power until it becomes actually available, which is disadvantageous.
Also in the conventional type of radio equipment, a level of pilot signal is measured, whether any received signal exists or not is determined, and then squelch open/close operation is executed. For this reason, it is difficult to distinguish a low power pilot signal from noise, and also malfunction of squelch easily occurs at a place with a high noise floor or in a weak electric field. Furthermore dispersion in sensitivity due to differences among individual radio equipment sometimes occurs during the manufacturing step, so that it is required to set up a threshold value used for determination of squelch for each piece of radio equipment, which increases cost.
Similarly in the conventional type of radio equipment based on the AFC system, a level of a signal is measured for making a determination as to whether the signal is a pilot signal or not, but when receiving a signal in a weak electric field, it is difficult to correctly distinguish a low power pilot signal from noise. Hence, sometimes an error may occur in measurement of the frequency of a pilot signal, which may in turn cause malfunction in the AFC.